Hammer WEIGHT / STROKE / SPRING RATE & power question & talks

[PBguns]

I have had excellent results with even much lower hammer weights....15g-19g.
Running really weak poppet spring and a pretty strong hammer spring.
I think my hammer being so lightweight compared to anyone else's....married to the really light poppet spring (my valve opens/shuts very quick).

[Motorhead]

I have had excellent results with even much lower hammer weights....15g-19g.
Running really weak poppet spring and a pretty strong hammer spring.
I think my hammer being so lightweight compared to anyone else's....married to the really light poppet spring (my valve opens/shuts very quick).

Agree ... in a typical purpose built gun,  My FT rigged 19 ft lb M-rod ran for years quite successfully on a 14 gram hammer. Tad more weight required with an SSG tho.
PCP's are odd little beasts that no sooner than you think you got em figured out they throw you a curve ball.

[PBguns]

I couldn't agree more Scott.
This thread is going to help a lot of members out with their tuning.
After all the hammer weight, stroke, and spring rate is what it's mostly about.....(just have to play a little with that balancing act between them).
Once you do.....it's a sweet thing!

[FuzzyGrub]

I have had excellent results with even much lower hammer weights....15g-19g.
Running really weak poppet spring and a pretty strong hammer spring.
I think my hammer being so lightweight compared to anyone else's....married to the really light poppet spring (my valve opens/shuts very quick).

Your results with that light of hammer have surprised me.  Especially given this is a 25 cal reg'ed at 2200psi.

So, it seems to be keyed off of the very light poppet spring.   That LW hammer weight seems more inline with a 177 reg setup, especially with ssg.  And that is with a much less than stock poppet spring.   I don't recall seeing much experimentation with the poppet spring strength/preload, probably because it is such a pita to change/adj. 

[rkr]

Bob,
Yes in that as Stroke increased in @ .100" increments and hammer as it sits against poppet deeper, the SSG as a "cartilage" assembly is simply moved further into the rear the same .100" incrementally maintaining the GAP as a constant.

Guess i need to remove hammer and look carefully at face of valve body abd see if the 3/8" striker is leaving a mark being telling of a collision  and subsequent rebound effects in play

Put a tiny ball of modeling clay on the valve body face. Once you fire the gun it will show you how close the hammer gets.

Edit: Bob already suggested that. Please remove.

[Buldawg76]

Scott
As Bob suggested you can use modeling clay or even some plastigage that is used to measure bearing clearance in engines when building as I believe he stated that also just used a different name for it.

Mike

[shorty]

Wouldn't it be silly if it was just the transfer port leaking through between the valve and the pressure tube.

On the SSG for it, I didn't see any pass through holes to allow for air compressing between the face of the hammer and the face of the valve.

Just another possibility ? 

[triggertreat]

Agree ... in a typical purpose built gun,  My FT rigged 19 ft lb M-rod ran for years quite successfully on a 14 gram hammer. Tad more weight required with an SSG tho.
PCP's are odd little beasts that no sooner than you think you got em figured out they throw you a curve ball.

These PCPs is keep us enthused and motivated and for some, eager to share their findings with others.  I for one enjoy tinkering with mine and sharing what I've found regardless of others shortfalls towards me.

[rsterne]

That's a really good thought, Tim.... If the space between the hammer and the back of the valve does not have sufficient venting, it can get pressurized from a leak around the valve stem, blowby aft from the transfer port, or just the movement of the hammer itself.... If Scott finds that the striker isn't hitting the back of the valve, he can look for those things while he's in there.... I've had that problem in a few PCPs year ago, just forgot about it.... I know Scott had it happen on his B-50/51 as well.... I actually had an early version of my Hayabusa go "machine-gun" on me because of a leaky valve stem, a good fit on the hammer, and no vent hole.... A 3/16" hole in the side of the tube right behind the valve cured it completely....

Bob

[Buldawg76]

I would think the trigger slot in the tube should be capable of venting the air in front of the hammer fairly easy but I may be wrong depending on how well the trigger housing is sealed when assembled. If the valve stem is loose enough to allow pressurized air past it in any great amount that certainly could be what's happening. its worth looking at indeed.

Mike

[Motorhead]

Well,
 While not having any clay handy, did remove hammer and place some Prussian blue onto strikers face.
Maxed out stroke and fired a few shots .... CONTACT  with face of valve body.

So the kinetic rebound theory likely my culprit here.

Now whats interesting and likely a LIGHT HAMMER cause & effect is that we're getting .180" of lift !!! WOW !!
But due to hammer weight being so low the valve is able to close super fast ( Bob S did some math on this within another thread )

The DWELL is so low and valves opening and closing so fast were not really getting to a point where DUMP shots are happening ... not even close it would seem

My read on this ( Past the reason for the velocity decrease we likely have a handle on ) is that we are operating at the extreme side of Low Dwell / High Lift where as at the other side we could use a HEAVY hammer with a light strike and get Low lift / High dwell 
It appears most PCP's likely operate somewhere in the middle +/- having modest lift and modest dwell.


** BTW,
Hammer used is well vented and has 4 lengthwise flutes of 1/2" radius @ .075" deep end to end. @ 50% of the contact area hammer has to the tube is air clearance.  This not only gives air a path to vent front to back, but also reduces frictional losses.

[Buldawg76]

So you have been giving that light hammer a real workout bouncing it back and forth off the valve and hammer spring.

Makes sense and another thing to keep in mind with custom tuning.

 

[rsterne]

Thanks for the confirmation, Scott.... always nice when the suggested answer turns out to be the correct one.... You are absolutely correct, the relationship between lift and dwell depends on the hammer weight, for a given spring and hammer stroke.... The average spring force (average of preload and cocked) and the travel are the governing factors in hammer energy, which affects the lift.... The weight of the hammer doesn't affect it.... however, changing the hammer weight changes the momentum, which changes the dwell.... Using a heavy hammer, for a given spring and travel, therefore increases the potential amount of air released, and hence the potential FPE....

Since the area under the lift/dwell curve governs how much air the valve allows to get to the pellet, and that is what governs the potential FPE, there are many combinations of lift and dwell that will produce the same FPE.... If you do it with a very light hammer, you get lots of lift and not much dwell.... If you do it with a heavy hammer, you gets lots of dwell and not much lift.... A light hammer will generally require a bit more spring force to make up in dwell what it loses in lift to produce the same FPE.... It appears that in this case, Scott's light hammer was running into the back of the valve, limiting the lift and dwell, and then rebounding faster as he continued to increase the hammer stroke, reducing the dwell and hence the velocity....

Bob